Quantum mechanics is a branch of physics that deals with the behavior of matter and energy at the atomic and subatomic level. It is based on the principles of quantum theory, which states that energy and matter can exist in discrete units, or quanta, and that these quanta can be used to explain the behavior of matter and energy on a very small scale. Quantum mechanics has been used to describe phenomena ranging from the behavior of atoms to the properties of materials. It has also been used to develop new technologies such as lasers, transistors, and computers.